The IEEE 802.11 WLAN is one of the most deployed wireless technologies and will play a major role in next-generation wireless communication networks. To share the wireless medium, the IEEE 802.11 standard defines two access methods at the medium access control (MAC) layer: the mandatory distributed coordination function (DCF) and the optional point coordination function (PCF).
DCF uses a carrier sense multiple access with collision avoidance (CSMA/CA) method to decide which station in the network should send the packets out. No coordination is proposed in DCF so many collisions occur, especially in high load WLANs. This causes the WLAN throughput to become worse.
In an infrastructure basic service set (BSS), PCF can be used and the access point (AP) keeps polling the non-AP stations to coordinate access to the wireless medium, but the realization of this is complicated because the AP has to poll the non-AP stations for the transmission of each polled packet. Furthermore, it is difficult to integrate an effective power saving mode into it.
In CSMA/CA, stations listen to the wireless medium to determine when it is free. Once a station detects that the medium is free, it begins to decrement its back-off counter. Each station maintains a contention window (CW) that is used to determine the number of slot times a station has to wait before transmission. The back-off counter only begins to decrement after the medium has been free for a DCF Inter-Frame Space (DIFS) period. If the back-off counter expires and the medium is still free, the station begins to transmit. Thus, it is possible that two stations may begin to transmit at the same time, in which case a collision occurs.
The collisions (or other transmission problems) are detected by the lack of an acknowledgement from the receiver. After the detection of a collision, the station randomly picks a new back-off period from its CW (the CW grows in a binary exponential fashion) and then attempts to gain control of the medium again. Due to collisions and the binary back-off mechanism, there are no transmit guarantees with DCF and, as described above, these problems are made worse by the lack of coordination available with the use of DCF.